I am sorry I do not have an answer for you. Will be watching this post closely because we are also considering two different floors in vinyl and how to make them work.

Pictures really help.

We did this two houses ago. It was a 1926 home with dark-stained oak hardwoods on the main floor. The butler's pantry/breakfast room had been taken into the kitchen years before and vinyl put down. When we pulled up the vinyl there was strip hardwood in the BR area, and a pine subfloor in the kitchen, all flush. So the kitchen was originally intended to have linoleum, I believe.

We went through various ideas and opted to install new heart pine in the whole kitchen/BR area. It was lighter, and with wider planks than the rest of the house. But heart pine was used in our area and in the era of the home, so it was at least a reference to something appropriate. Because of the thickness of the planks, there was an eased 3/4" step up. It turned out fine, and we really liked it.

I don't have a picture that shows both floors, but here is a not-so-great picture of the kitchen (when the people who bought it from us were selling it):